# Bicycle Generators

Discussion in 'The Projects Forum' started by bleopold, May 15, 2014.

1. ### bleopold Thread Starter New Member

May 15, 2014
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Hello,
I am trying to build a bicycle generator with my Physics students and am using a 1 wire Delco alternator. We have a gear ratio or mechanical advantage of about 60 to 1 when you include the bicycle gears and the large pulley(Rear wheel and alternator pulley. We are able to create a voltage of 15 to 16 volts once the alternator reaches the cut in speed(self energizing) However when put a load on the circuit it becomes very hard to pedal. We hooked a 40 watt lamp up to the circuit directly through a 400 Watt inverter. It lights for a brief time but then becomes impossible to pedal. We put a resistor running from positive of alternator to positive of inverter but bulb does not light as I think current is too low. Any suggestions would be appreciated.

Thank you,

Brian

2. ### alfacliff Well-Known Member

Dec 13, 2013
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a good demonstration of how making electricity take energy. if you use a 40 watt 12 volt bulb it will be easier to pedal, demonstrating the efficiency of the invertor.
if you want to compare how much "horsepower" each student can produce, put a speedometer on the bike and a fixed load.

3. ### #12 Expert

Nov 30, 2010
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Nothing is broken. You're just experiencing the physics of generating electrical power.

A car alternator is designed to use up more than a horsepower, and humans can only generate about 1/4 HP for short bursts of time. Perhaps a smaller alternator is the answer? Trying to crank an alternator that is designed for several hundred watts probably requires a lot more energy than trying to crank one that is designed for 40 or 50 watts.

I'm saying the scale of your parts is excessive for the scale of your load and that probably introduces energy losses that are larger than the load.

4. ### alfacliff Well-Known Member

Dec 13, 2013
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have you ever had a bicyele with a generator powered light on it? it is harder to power when you turn it on too. and that just a little 6 or 12 volt low power bulb. it takes torque (horsepower) to make an alternator or generator to produce electricity.

5. ### k7elp60 Senior Member

Nov 4, 2008
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It takes 1 horsepower to generate about 750 watts of power. I once did nearly the same thing as bleopold did about 10 years ago. I actually used a generator
as they have enough residual magnetism in the coils to start generating power, whereas most alternators require dc voltage from the battery to start working.

I even used a 50 watt 12 volt bulb connected to the vehicle generator. I could only light the lamp for a very short time until I was physically exhausted.

Jul 18, 2013
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I assume you mean 1 wire and frame common (OWK as ground) which usually indicates a built in regulator?
If you want to instruct students on the workings and to control the excitation manually or externally, locate the two wires to the field and feed a varying DC level into them, you could start off with self excitation if it supports it, otherwise you would need a separate DC into the two field wires initially, these are the two that go to the slip rings.
I believe that with full 12v excitation an alternator will output up to 50v off load.
Max.

7. ### Austin Clark Member

Dec 28, 2011
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As others have stated, this is normal and expected.
Both as a generator and a motor, current is proportional to torque. This is because current within the motor/generator creates an electromagnet that pushes and pulls against the rotation of the rotor.
Without a load, there is no current and therefore no torque is necessary to turn the rotor (other than to overcome friction). The greater the load, the greater the current, and therefore the greater the torque.
Voltage is proportional to RPM.

This also makes sense because Power = Voltage * Current = RPMs * Torque

Last edited: May 15, 2014
8. ### AnalogKid Distinguished Member

Aug 1, 2013
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Faraday''s Law: moving a wire through a magnetic field induces a current into the wire. Ampere's Law: moving a current through a wire generates a magnetic field. The problem is that Faraday's Law has a minus sign in it, so the field produced by the current opposes the field that produced the current. The force on the wire caused by the two opposing fields is called the Lorentz Force.

ak

Austin Clark likes this.
9. ### Brownout Well-Known Member

Jan 10, 2012
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Can you try a ~4W, 12V bulb? Find one at your local auto parts store, say a tail or license plate light. I would think it would be possible for a person to make 40W, but I might be mistaken. Try a lower wattage bulb and see what happens.

10. ### Austin Clark Member

Dec 28, 2011
409
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An incandescent bulb probably isn't the best load to use. An LED would be better, or perhaps another motor? Another motor is what I'd use. If it's big enough, you can even put a load on it and FEEL the increase in torque on the generator! It's really neat. It's like a magic invisible transmission.

A bulb is really bad for this because it has incredibly low resistance (meaning it will draw a ton of current, almost acting like a short) until it heats up. This means it will be hard to get the pedals moving at first.

11. ### bleopold Thread Starter New Member

May 15, 2014
2
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We were able to get a lower incandescent bulb and light it indefinitely or least until rider tired. Also we were able to play music through speakers. I think if I were to do this again it may be easier with a DC motor.. Your thoughts?

12. ### Brownout Well-Known Member

Jan 10, 2012
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I think you did the right thing. Watching a lamp illuminate from a mechanical input captures a child's imagination.

13. ### crutschow Expert

Mar 14, 2008
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I agree with Austin. Use a 12V string of high efficiency LEDs. That will give much more light for a given input and be a better measure of the amount of energy needed to generate visible light.. With an incandescent bulb most of the energy (>90%) is going into heat (invisible IR output from the filament).

14. ### Sircut5 New Member

Jan 23, 2014
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I like to be somewhat destructive on a lot of electronics and took apart a few microwaves. The turntable motors are great generators and I hooked one up to a small led and turned it by hand. I was measuring without leds and voltage was in hundreds of volts and very low current. When connected to load or just a few (30?) milli amps the led lighted and rotating became very hard. I could hardly believe the difference it was almost like it was seizing but removing the load freed it up!

Jul 18, 2013
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The principle behind motor dynamic braking.
Max.

16. ### inwo Well-Known Member

Nov 7, 2013
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Use an external regulated alternator or gut this one. Full fielding it will allow it to light a 120vac bulb directly.
Take "power out" from stator. With or without rectifiers.
It's not that much effort to light a 40 watt bulb. I've hand cranked a telephone magneto to a glow.

17. ### wayneh Expert

Sep 9, 2010
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Yes and no. An athlete can deliver ~200 watts to the pedals for an extended time. A bicycle is very efficient at converting this to motion, but I suspect, as #12 has noted, that the efficiency of running an automotive alternator as the OP has described is much lower. It's not the right match for the job.

Lighting that 40W bulb might require 200W or more at the pedals, and this would be tough for a non-athlete. Humans are about 25% efficient, so delivering 200W requires 800W of metabolism, or about 680 calories per hour.

I can do that because I work at it, but most folks I know are couch potatoes that cannot sustain that rate for more than a few minutes.

18. ### inwo Well-Known Member

Nov 7, 2013
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Math isn't always the answer. So I'll go for the "yes" on that one.

It only takes a few watts to begin lighting a 40 watt bulb from an unregulated source. Such a full fielded alternator.

As I stated previous. I've actually done it with a small hand cranked magneto. Not a huge effort.

So IMO, a 40-100 watt incandescent makes a great demo of effort vs output.

19. ### ErnieHorning Member

Apr 17, 2014
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We have a bicycle generator like yours set up at the Science Museum. There’s a switch on the output that you can choose an AM radio, a 15 watt florescent bulb (60 watt equivalent), a 40 watt light bulb and a black & white TV. It shows kids how much energy it really takes to power different electronic devices. The radio is a piece of cake and is not much different then riding a bike. The TV takes so much energy that you can only pedal for a few seconds. It’s part of an energy conservation display. It’s also a bit of an ego thing for boys too. LOL

20. ### wayneh Expert

Sep 9, 2010
12,148
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I don't disagree. My point, not well made, was to agree with #12 that the problem is the equipment mismatch.

Difficulty lighting a 40W bulb for a short time is evidence that something is wrong. People usually just hop on these things for a few seconds, and for that short time you should be able to light up even a 500W bulb.

I've seen the load on these things being a series of light bulbs that light up like a bar graph in proportion to the effort. Like ringing the bell with the hammer at the fair. I'm not sure how they do that with incandescent bulbs - maybe an LM3914 based circuit controlling relays?